This invention relates to an electrode for medical applications, and more particularly to an implantable cardiac cardioversion/defibrillation electrode.
Electrodes implanted in the body for electrical cardioversion or defibrillation of the heart are well known. More specifically, electrodes implanted in or about the heart have been used to reverse (i.e., defibrillate or cardiovert) certain life-threatening cardiac arrhythmias, where electrical energy is applied to the heart via the electrodes to return the heart to normal sinus rhythm. See, for example, commonly assigned U.S. Pat. No. 4,291,707 to Heilman, relating to a planar patch defibrillation electrode, and pending U.S. patent application Ser. No. 07/334,652, entitled Cardiac Defibrillation/Cardioversion Spiral Patch Electrode, filed Apr. 10, 1989, now U.S. Pat. No. 5,052,407.
The Heilman patent specifically discloses an implantable cardiac electrode comprised of a planar conductive material insulated completely on one side and partially on its other side. Apertures are provided around the insulated perimeter of the partially insulated side of the electrode to provide for efficient and uniform energy transfer to the heart tissue by eliminating the so called "edge-effect".
The pending application Ser. No. 07/334,652 relates to a spiral patch electrode comprised of an elongated conductor preformed to adapt a spiral planar patch configuration when deployed on or about the heart surface.
The amount of energy delivered by the electrodes to the heart during defibrillation (or cardioversion) depends on the placement of the electrodes and the ability of the electrodes to distribute the energy uniformly throughout a major portion of the heart. This energy is called the defibrillation or cardioversion energy.
For purposes of the following discussion, no distinction will be made between cardioversion and defibrillation, although the respective energy levels and timing sequences may differ. Both will be referred to as defibrillation.
A problem with many defibrillation electrodes is that they fail to provide a uniform current discharge distribution. Specifically, high current densities occur near the edges or perimeter of the electrode which damages underlying tissue. In addition, the high current densities along the perimeter of the electrode result in a large potential drop near the electrode which greatly reduces the voltage gradient within the myocardial mass. Therefore, the effectiveness of the defibrillation pulse is hindered.
Some attempts have been made to obtain more uniform current distribution on externally applied electrodes for external pacing or defibrillation. One such device comprises an externally applied patch system for defibrillation which reduces the potential for skin burning during external defibrillation.
This approach involves the use of various resistances to force a greater percentage of total current towards the center of an externally applied electrode. However, in the field of implantable devices, the amount of energy available is finite. Consequently, implantable defibrillation electrodes must provide a uniform current distribution with minimal interface impedances.